Pipes for use for example as gas and water mains are commonly made by extrusion of plastics material such as nylon, high density polyethylene, polypropylene or polyvinyl chloride.
Much attention has been directed to improving methods for connecting together plastic pipes and particularly to methods enabling connection of such pipes after a major portion of one of them has been buried in the ground, as occurs for example during the installation of a new mains pipeline or replacement of a section of an existing pipeline. Attention has also been directed at reducing the level of skill required to make such connection.
One method of connecting such pipes involves the use of fittings known variously as "electric fusion fittings" or "electric welding fittings".
A typical electric fusion fitting comprises an injection moulded pipe collar the inside diameter of which is sized to sleeve the end portion of each of two pipes to be joined together. The collar is made of substantially the same plastics as the pipes and has a helix of wire moulded integrally therewith, the wire helix being coaxial with the collar and disposed at the inner surface of the collar or embedded within the collar adjacent the inner surface.
In use, one pipe end portion is inserted into the collar from one collar end and another pipe end portion is inserted into the collar from the other collar end so that the two pipes are substantially in abutment and sleeved at their respective end portions by the collar. The wire helix of the collar is thus disposed substantially at the interface between the collar inner surface and the outer surface of the respective pipe end portions. The wire helix is connectable, typically via terminal means moulded into the collar, to an electric welding circuit which provides a pulse of electric current of predetermined characteristics through the wire coil thereby heating the coil and causing fusion of the plastics material adjacent the helix surfaces, that is to say, causing the plastics of the collar to fuse with the plastics of the respective pipe end portions.
The result is that the pipe end portions are welded or fused together with the collar.
In the past electric fusion fittings have been manufactured by winding wire as a helix onto a mandrel and then forming the plastics collar around the coil and mandrel by injection moulding of the plastics.
A separate mould has thus been needed for each size and shape of fitting. As a result the cost of fusion fittings has been so high as to inhibit their widespread use, despite acknowledged advantages of the effectiveness and simplicity of that method for pipe jointing.
An objective of the present invention is to provide a method of manufacture of electric fusion pipe fittings which avoids the need for injection moulding about a wire helix and which in preferred embodiments would enable such fittings to be manufactured at lower cost than when manufactured in accordance with prior art methods.